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// Add your own function to filter the fields
add_filter( 'submit_job_form_fields_get_user_data', 'custom_submit_job_form_fields' );

// This is your function which takes the fields, modifies them, and returns them
// You can see the fields which can be changed here: https://github.com/mikejolley/WP-Job-Manager/blob/master/includes/forms/class-wp-job-manager-form-submit-job.php
function custom_submit_job_form_fields( $fields ) {

    // Here we target one of the job fields (job_title) and change it's label
    $fields['job']['application']['value'] = 'Enter an email address';

    // And return the modified fields
    return $fields;
}